TF2 Cosmetic: Help me with rigging!

I made a backpack for engineer:

[img]http://s5.postimg.org/qqobg2tqf/sentry.png picture upload sites[/IMG]
I need to rig the straps (I already rigged the sentry, clasps and blueprint).

Help me. I using a Blender 2.73 and I working on default engineer skeleton.